Support for ES6/ES2015 ECMAScript Modules

[JoshRodd]

I'm planning to implement support for ES6/ES2015 style JavaScript Modules (commonly known as "ESModules" or "ECMAScript Modules"). The purpose of this issue is to track the progress / share with anyone else who may be working on the same thing or wants the feature/wants to test it.

Overall:

• Implement a standard V8 module callback that then reaches into the mini-racer Python module.
• Add some functions to mini-racer to be called to resolve modules.
• Implement a module resolver; in its simplest incarnation, this would just look for a .js (or .mjs) file in the current directory.
• More sophisticated module resolvers could be built and enabled.
• Module resolution will vary considerably from how a typical web browser works.

A simple implementation would be able to do this:

sample_module.mjs:

export function my_func() { return 123; }

use_module.py:

from py_mini_racer import MiniRacer

ctx = MiniRacer()
ctx.import_module('my_func', './sample_modules.mjs')
print(f"Should be 123: {ctx.call("my_func")}")

[declann]

+1 I'm looking at options to bring calculang directly into Python and this support would be critical - the JS ecosystem is full-steam ESM now!

[bpcreech]

This is interesting!

I guess such a thing would need to implement a module import hook, like this one in d8 (the v8 debugger).

I think in order to keep sandboxing intact while providing extensibility, I think we'd need to expose a callback interface in Python so that the py_mini_racer user could control where the module source comes from, rather than, e.g., directly reading from the filesystem. The JS module system is async (at least for dynamic imports) so I think this could be relatively feasible using asyncio, like this:

async def my_module_loader(origin: str) -> str:  # exact signature TBD
    return Path(origin).read_text()  # do something smarter here

async def run_with_external_modules() -> None:
    with MiniRacer() as ctx:
        async with ctx.module_source(my_module_loader):
            ctx.eval("await import('some_module.js')"